drop getLastElement() function
fix expected test results
strip shared oid prefix from key

That's a bit useless to index result by the full oid, whereas most
following operations will first try to get rid of it, so let's strip
directly:
$snmp->walk('1.2.3')
returned    { '1.2.3.1' => 'A', '1.2.3.2' => 'B', '1.2.3.3' => 'C' }
now returns { '1'       => 'A', '2'       => 'B', '3'       => 'C' }